sip.conf:

[demo-bob]
type=friend
host=dynamic
secret=secret567
context=users
insecure=very

[isp]
type=friend
host=10.10.10.10
fromuser=demo-bob
qualify=no
context=users
insecure=very

extension.conf:

[users]
exten=>_99.,1,Dial(SIP/${EXTEN:2}@isp)

Клиент регится, пытаюсь звонить:

[Sep  6 15:05:54] NOTICE[8906]: chan_sip.c:24933 handle_request_subscribe: Received SIP subscribe for peer without mailbox: demo-bob
  == Using SIP RTP CoS mark 5
    -- Executing [99100@users:1] Dial("SIP/demo-bob-00000000", "SIP/100@isp") in new stack
  == Using SIP RTP CoS mark 5
    -- Called SIP/100@isp
[Sep  6 15:05:56] NOTICE[8906]: chan_sip.c:20425 handle_response_invite: Failed to authenticate on INVITE to '"demo-bob" <sip:demo-bob@10.10.10.100>;tag=as38288baa'
    -- SIP/isp-00000001 is circuit-busy
  == Everyone is busy/congested at this time (1:0/1/0)
    -- Auto fallthrough, channel 'SIP/demo-bob-00000000' status is 'CONGESTION'

В чём проблема? Транк с той стороны без паролей и т.д.